Toyota Repair: Frozen Spark Plug in Toyota MR2, 1992 toyota mr2, toyota mr2

Question
I have recently bought 1992 Toyota MR2 with the 2.2 liter OHC naturally-aspirated 4-banger, has 136k miles on it and in great shape. I decided to change the spark plugs to those Bosch 4-prong platinum plugs because in my experience they DO make a sharply noticeable difference in power and eagerness to rev. I successfully changed 3 of them. But one plug, no matter how many rotations of the ratchet, remained as stuck as if it was the first try. Usually after the first yank, plugs loosen right up. It did move, and I probably ended up rotating it 2 or 3 times, but it remained amazingly hard to turn. I was afraid it was stripped or the threads in the block were stripped or for some other reason it was permanently rusted in there. It really felt as if I was going to snap it in half and then I'd be really screwed. I sprayed a lot of Liquid Wrench around it and let it eat away for a day or so. Made no difference. Still as tight as the first try.

Is there a chance I could snap the spark plug in half? Is there a chance that the block's threads could be stripped? I feel like I should leave well enough alone and let the engine ride with this old plug forever, fearing I'll create a far worse problem if I mess with it any further.

BUT, now that there are 3 high-performance new plugs and 1 old standard Autolite-type plug in the engine, there's a new "thrumming" to the idle as if the balance was thrown off by one cylinder not getting the equivalent combustion of the other cylinders, and the engine vibrates more at idle. Plus, the plug is "withdrawn" by a millimeter or two, which changes the properties of the combustion chamber relative to the others. But it still feels as tight as if it were bolted down, so I'm sure it's not a loss of compression. I'd hate to see this imbalance/vibration cause a greater problem in the long run.

Your thoughts?  

Answer
It sounds like that spark plug is damaged already to the point where it won't fire and the engine is running on three cylinders, the bottom line is that this spark plug is seized in the cylinder head and the threads will have to be repaired once the plug has been removed, the chances of the plug breaking off are small, you will have to try to remove the plug and it will probably pull the threads out and another plug won't tighten, the solution is to tap the spark plug hole oversize and then install a steel insert, this can be done without removing the cylinder head but this is better left to a repair shop because of the special tools involved.